tests/installed: support TESTS filter

Lifted from rpm-ostree. Makes iterating on a single test much faster.
Example use:

    TESTS=label-selinux ./ostree/tests/installed/run.sh

Closes: #1442
Approved by: cgwalters

tests/installed/run.sh

@@ -4,6 +4,14 @@ set -xeuo pipefail
 
 dn=$(dirname $0)
 for tn in ${dn}/itest-*.sh; do
+    if [ -n "${TESTS+ }" ]; then
+      tbn=$(basename "$tn" .sh)
+      tbn=" ${tbn#itest-} "
+      if [[ " $TESTS " != *$tbn* ]]; then
+        echo "Skipping: ${tn}"
+        continue
+      fi
+    fi
     echo Executing: ${tn}
     ${tn}
 done